The present invention relates to a laser beam receiving unit used in the case of detecting whether flying fine particles pass or not by allowing the flying fine particles, liquid droplets and the like to intercept the fine beam of a laser light to detect a reduction in the strength of the laser beam, for instance, like a laser sensor used to detect ink droplets of an ink jet printer, and, particularly, to a laser beam receiving unit which can detect fine particles exactly at a high response rate even if the speed of the fine particles intercepting the laser beam is high.
Conventional laser beam receiving units include those which, as shown in FIG. 3, converge a laser light 31 which propagates through a space by a condenser lens 32 and receives the laser light 31 by a light receiving element 33, e.g. a photodiode or those which, as shown in FIG. 4, converge a laser light 31, emitted from the end face of an optical fiber 35, by a condenser lens 32 and receives the laser light 31 by a light receiving element 33, e.g. a photodiode.
Laser light which propagates as parallel light as shown in FIG. 3 is received by a photodiode 33 disposed at the position of the focal distance f of the condenser lens 32. Also, as shown in FIG. 4, when incident light is not parallel light, a photodiode 33 is disposed at the position sxe2x80x3 determined by the Gauss""theorem (the formula of a convex lens) given by the following formula:
1/s+1/sxe2x80x3=1/f
provided that the distance between the end face of an optical fiber 35 and the opposed surface of the condenser lens 32 is s and the distance between the backface of the condenser lens 32 and the light receiving element 33 is sxe2x80x3.
In the case where the laser beam is once converged in a space, fine particles and the like are detected at a narrow part of the beam diameter and thereafter the beam which rediverges is received, the receiving element is also disposed at the position sxe2x80x3 determined by the aforementioned Gauss""theorem.
The condenser lens 32 is formed of an aspheric lens and formed so that the laser beam converges at the aforementioned determined position sxe2x80x3 even though the beam is incident on an end side of the condenser lens eccentrically and convergent point is off the center line, the optical axis of which is inclined relative to the condenser lens. Because of this reason, a relatively large light receiving element is formed, which makes it possible to receive light with the best sensibility without any trouble even if there exists a little eccentricity on the side of the laser source.
As outlined above, the conventional light receiving unit such as those for laser sensors is designed to be disposed on the convergent point of the condenser lens to raise the sensibility of the light receiving element sufficiently. While, as will be mentioned later, the inventors of the present invention have found that as for a light receiving portion which detects high speed fine particles, if the area of a light receiving element is increased, the capacity of the light receiving portion is increased and the responsivity of the light receiving portion is reduced. It is therefore necessary to decrease the area of the light receiving element. However, if the area of the light receiving element is decreased, there is the problem that when the beam incident on the light receiving unit is eccentric, the laser beam is not incident on the light receiving element and hence there is the case where the laser beam cannot be detected at all.
The present invention has been conducted to solve such a problem and it is an object of the present invention to provide a light receiving unit used to detect a laser beam, the light receiving unit being able to always receive light regardless of a little eccentricity of a laser source while the response speed is raised.
The inventor of the present invention has earnestly investigated the reason why the variation of fine particles can not be detected sufficiently when flying fine particles such as droplets of an ink jet printer are detected by a conventional laser sensor and, as a result, found that the reason is that a light receiving element can respond only insufficiently to the variation of the fine particles. Then the inventor has conducted further investigations and, as a result, found that if the area of the light receiving element is large, the capacity of the light receiving element is increased, the response speed becomes low and high speed fine particles cannot be detected sufficiently if the area is designed to be 2 mm2 or less. The inventor has also found that although the case where an eccentrically incident beam can be received insufficiently arises when the area of the light receiving element is small, even the eccentric beam can be received sufficiently by locating the light receiving element at a position before the convergent point (image point).
A laser beam receiving unit according to the present invention, which receives the laser beam to detect a detection object passing through the laser beam comprising: a condenser lens which converges the laser beam and a light receiving element which receives the incident beam converged by the condenser lens, wherein the light receiving element is formed, such that the light receiving part thereof has an area of 2 mm2 or less, and disposed at the side of the condenser lens than the point where the beam is converged by the condenser lens and at the position more apart from the condenser lens than the position of the focal distance of the condenser lens.
Here, the point where the beam is converged, that is convergent point means the point at which the beam spread from a light source is converged by the condenser lens and form an image.
This structure ensures that because the condenser lens made of an aspheric lens is formed so that also beam which is eccentrically incident crosses the center line of an optical system and converges on a plane positioned at the same distance from the lens as that of the convergent point of the beam emitted from a non-eccentric light source, a portion where the eccentric beam crosses the non-eccentric beam is formed closer to the condenser lens to the convergent point. As a consequence, by shifting the light receiving element to the side of the condenser lens as far as the portion where the beam from a light source having the largest allowable eccentricity crosses the center line of the condenser lens and the light receiving element, beam from any eccentric light source is incident on the light receiving surface of the light receiving element.
When the focal distance of the condenser lens is f, the distance between the point where the laser beam is converged by the condenser lens and the condenser lens is sxe2x80x3 and the distance between the position where the light receiving element is disposed and the condenser lens is x, preferably the light receiving element is disposed at the position determined by the formula f xe2x89xa6xc3x97xe2x89xa6(sxe2x80x3+f)/2.
The size of the light receiving element is preferably a 1 mm by 1 mm square or less,and more preferably a 0.75 mm by 0.75 mm square or less to raise the response speed.
The laser beam is, for instance, a beam in which the laser light emitted from a laser source is converged by the condenser lens to form a minimum beam spot portion, and a part of which is intercepted by a detection object passing through the minimum beam spot portion is detected by the light receiving unit.
When the detection object is high speed fine particles, specifically, ink droplets of an ink jet printer, the consumption of ink can be detected exactly and hence the invention has a high effect.